COMMERCE CITY — Three years after a former weapons and pesticides plant reopened as the nation's largest urban wildlife preserve, bison are multiplying too fast.

There are 85 today, more than quadruple 2007's number, threatening to degrade drought-prone prairie at the Rocky Mountain Arsenal National Wildlife Refuge. Federal biologists say they must cut the herd by 25 — and keep it at 60 until fenced habitat is expanded.

A roundup is planned for Dec. 17, and the biologists are looking for options short of slaughter, such as trucking the animals to other bison preserves.

The forced thinning reflects a growing challenge in restoring wild species as the U.S. population booms in the West: finding suitable open space. Bison once numbered 20 million and roamed North American grasslands before they were hunted to near extinction. Today, the federal government's Bison Conservation Initiative aims for a wild population of fewer than 10,000 nationwide.

The thinning also highlights a problem for Colorado, where growing numbers of people are drawn by wild open space. Intensifying efforts to profit on those people encroach on the space.

"Human constraints" are the main obstacle in trying to bring back bison, said Dave Lucas, a U.S. Fish and Wildlife Service biologist managing the refuge. "There are very few places in the United States where bison will be free-roaming. We have roads, yards, fences and people. Our country is not that wild anymore."

Northeast of Denver, master plans to ramp up commercial development on prairie around the refuge call for construction of retail shops, offices, hotels and housing. Denver Mayor Michael Hancock on Friday said development along Peña Boulevard — which leads to Denver International Airport, east of the refuge — "is inevitable."

Denver leaders propose a revenue-generating "aerotropolis." In Commerce City, west of the refuge, leaders also plan buildings to boost business.

Bison graze at Rocky Mountain Arsenal National Wildlife Refuge on Thursday. At top, tourists enjoy getting a closer look at the animals. Federal officials
Bison graze at Rocky Mountain Arsenal National Wildlife Refuge on Thursday. At top, tourists enjoy getting a closer look at the animals. Federal officials said the refuge has attracted 300,000 visitors this year, mostly to see the bison. That's eight times the visitation in 2011, prompting plans to accommodate 1 million visitors at the 27-square-mile refuge. International tourists in rental cars from DIA often stop first at the refuge, staffers said. (RJ Sangosti, The Denver Post)

Yet the bison themselves have become a driving economic-development force.

Federal officials said the refuge has attracted 300,000 visitors this year, mostly to see bison. That's eight times the visitation in 2011, prompting plans to accommodate 1 million visitors at the 27-square-mile refuge. International tourists in rental cars from DIA, Germans in particular, often stop first at the refuge, staffers said.

Hancock elaborated, in a prepared statement, calling the refuge "a great example of how the metro area balances being the fastest-growing region in the United States and protecting our natural resources like the bison."

"(The refuge) will be part of the aerotropolis planning as we leverage local assets, like the refuge, to make this area something that we can all be proud of," he said.

For example, Denver's plans for development on the 53-square-mile airport area include a viewing station along Peña, where drivers could pull off and see bison at the eastern edge of the federal refuge. A double fence would keep the bison in.

Gov. John Hickenlooper has supported transplanting bison to the refuge perimeter so people could see them. He has supported installation of signs to promote the refuge, bison and other wildlife.

Although the herd will be cut, refuge plans call for eventually putting in more internal fencing — and irrigating short-grass prairie — to expand the bison range to 12,000 acres from 2,600 today.

A motorist passing through the Rocky Mountain Arsenal National Refuge on Thursday stops to let a bison cross. The refuge is home to 85 bison, more than
A motorist passing through the Rocky Mountain Arsenal National Refuge on Thursday stops to let a bison cross. The refuge is home to 85 bison, more than quadruple 2007's number. (RJ Sangosti, The Denver Post)

That would allow 210 bison roaming right to the edge of Peña, Lucas said. But reaching that level may take a few years. Each bison eats about 50 pounds of grass a day, and overgrazing would weaken the refuge capacity to sustain raptors, prairie dogs, deer and coyotes.

Federal land managers are making master plans, too, for "strategic growth of the national refuge system" — which they say would require collaboration with cities and the state.

The Rocky Mountain Arsenal once ranked as some of America's most polluted land, a complex of 250 buildings at Denver's northeastern edge where the Army made chemical weapons and, later, Shell Oil made pesticides such as DDT. The cleanup took years. A test in 2007 brought 19 bison to see whether they could survive. Then, in September 2010, the Environmental Protection Agency transferred a final 2,500 acres to the U.S. Fish and Wildlife Service, clearing the way to open the refuge.

The bison proved resilient and thrived, with females producing about 18 new calves each spring. Ten males weigh more than a ton. They roll on their backs scratching in the sun, churning up clouds of dust.

A road enables close inspection. Some bison approach cars. A few years ago, two fighting males rolled against and out under a fence. Refuge staffers herded them back in through gates.

Last month, a male bison gored a female. Refuge managers, after conducting a necropsy to confirm no disease, marveled at how coyotes picked the carcass clean to the bones in two weeks.

That's evidence of a healthy ecosystem with wildlife functioning as they would in nature, Lucas said.

"No other city has what we have," he said. "All the leaders in this area need to sit down and figure out what the appropriate balance should be for open-space conservation and development as metro development moves east."

Bruce Finley: 303-954-1700, twitter.com/finleybruce or bfinley@denverpost.com